1. Field
This disclosure relates generally to integrated circuits and, more specifically, techniques for electromigration stress determination in interconnects of an integrated circuit.
2. Related Art
Electromigration refers to a process of diffusion of metal atoms due to mechanical stress created via electrical current flowing through one or more interconnects. This results in formations of voids and hillocks at low potential nodes and high potential nodes, respectively, and can lead to resistance increases and circuit failures. In general, electromigration should be considered in applications where relatively high direct current densities are expected, such as in microelectronics and related structures. As structure sizes in integrated circuits (ICs) decrease, the practical significance of electromigration increases. Electromigration first became of practical interest when the first ICs became commercially available. Electromigration research in the integrated circuit (IC) field began at a time when metal interconnects in ICs were about ten micrometers (microns) wide. Currently, IC interconnects are hundreds to tens of nanometers in width. Electromigration decreases the reliability of ICs (e.g. chips) and can cause the eventual loss of connections or failure of a circuit.
Although electromigration damage ultimately results in failure of an affected IC, initial symptoms may include intermittent glitches that are challenging to diagnose. As some interconnects fail before other interconnects, a circuit may exhibit random errors that are indistinguishable from other failure mechanisms. In a laboratory setting, electromigration failure may be viewed with an electron microscope, as interconnect erosion leaves visual markers on metal layers of an IC. With increasing IC miniaturization the probability of IC failure due to electromigration increases, as both power density and current density increase as IC size decreases. In advanced semiconductor manufacturing processes, copper has replaced aluminum as the interconnect material of choice, as copper is intrinsically less susceptible to electromigration. In modern consumer electronic devices, ICs rarely fail due to electromigration effects, because proper semiconductor design practices incorporate the effects of electromigration into the IC layouts. That is, nearly all IC design houses use electronic design automation (EDA) tools to detect and correct electromigration problems at the transistor layout-level.
In general, the “Blech length” has been used to denote a length limit for an interconnect below which electromigration will not occur at a current density. That is, any interconnect whose length is below the “Blech length” will not typically fail due to electromigration at lower current densities, but may fail at higher current densities. In general, an interconnect whose length is below the “Blech length” experiences a mechanical stress build-up that causes a reverse migration process that reduces or even compensates for material flow.